microwave heating processes involving carbon...

Final version published in Fuel Processing Technology, 2010, 91 (1), 1-8

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Microwave heating processes involving carbon materials

J.A. Menéndez*, A. Arenillas, B. Fidalgo, Y. Fernández, L. Zubizarreta, E.G. Calvo,

J.M. Bermúdez

Instituto Nacional del Carbón, CSIC, Apartado 73, 33080 Oviedo, Spain

Abstract

Carbon materials are, in general, very good absorbents of microwaves, i.e., they are

easily heated by microwave radiation. This characteristic allows them to be transformed

by microwave heating, giving rise to new carbons with tailored properties, to be used as

microwave receptors, in order to heat other materials indirectly, or to act as a catalyst

and microwave receptor in different heterogeneous reactions. In recent years, the

number of processes that combine the use of carbons and microwave heating instead of

other methods based on conventional heating has increased. In this paper some of the

microwave-assisted processes in which carbon materials are produced, transformed or

used in thermal treatments (generally, as microwave absorbers and catalysts) are

reviewed and the main achievements of this technique are compared with those obtained

by means of conventional (non microwave-assisted) methods in similar conditions.

Keywords: Microwaves, carbon materials, coal, catalysis, pyrolysis

*Corresponding author. Fax: +34 985118972. E-mail address: [email protected] (J.A. Menéndez)


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1. Introduction to the microwave heating of carbons

Microwaves lie between infrared radiation and radiowaves in the region of the

electromagnetic spectrum. More specifically, they are defined as those waves with

wavelengths between 0.001 and 1 m, which correspond to frequencies between 300 and

0.3 GHz. The microwave band is widely used in telecommunications. In order to avoid

interference with these uses, the wavelengths of industrial, research, medical and

domestic equipment are regulated both at national and international levels. Thus, the

main operating frequency in the majority of countries is 2.450 (+/- 0.050) GHz [1, 2].

Dielectric heating refers to heating by high-frequency electromagnetic radiation,

i.e., radio and microwave frequency waves. The interaction of charged particles in some

materials with the electric field component of electromagnetic radiation causes these

materials to heat up. The heat resulting from this interaction is mainly due to two

different effects. In the case of polar molecules, the electric field component of the

microwaves causes both permanent and induced dipoles to rotate as they try to align

themselves with the alternating field (2450 million times per second). This molecular

movement generates friction among the rotating molecules and, subsequently, the

energy is dissipated as heat (Dipolar Polarization). This is the case of water and other

polar fluids. In the case of dielectric solid materials with charged particles which are

free to move in a delimited region of the material, such as π−electrons in carbon

materials, a current traveling in phase with the electromagnetic field is induced. As the

electrons cannot couple to the changes of phase of the electric field, energy is dissipated

in the form of heat due to the so called Maxwell-Wagner effect (Interfacial or Maxwell-

Wagner Polarization) [1, 2].


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The materials which interact with microwaves to produce heat are called microwave

absorbers. The ability of a material to be heated in the presence of a microwave field is

defined by its dielectric loss tangent: tanδ = ε”/ ε’. The dielectric loss tangent is

composed of two parameters, the dielectric constant (or real permittivity), ε’, and the

dielectric loss factor (or imaginary permittivity), ε”; i.e., ε = ε’ – i ε’’, where ε is the

complex permittivity. The dielectric constant (ε’) determines how much of the incident

energy is reflected and how much is absorbed, while the dielectric loss factor (ε”)

measures the dissipation of electric energy in form of heat within the material [1, 2]. For

optimum microwave energy coupling, a moderate value of ε’ should be combined with

high values of ε’’ (and so high values of tanδ), to convert microwave energy into

thermal energy. Thus, while some materials do not possess a sufficiently high loss

factor to allow dielectric heating (transparent to microwaves), other materials, e.g. some

inorganic oxides and most carbon materials, are excellent microwave absorbers. On the

other hand, electrical conductor materials reflect microwaves. For example, graphite

and highly graphitized carbons may reflect a considerable fraction of microwave

radiation. In the case of carbons, where delocalized π-electrons are free to move in

relatively broad regions, an additional and very interesting phenomenon may take place.

The kinetic energy of some electrons may increase enabling them to jump out of the

material, resulting in the ionization of the surrounding atmosphere. At a macroscopic

level, this phenomenon is perceived as sparks or electric arcs formation. But, at a

microscopic level, these hot spots are actually plasmas. Most of the time these plasmas

can be regarded as microplasmas both from the point of view of space and time, since

they are confined to a tiny region of the space and last for just a fraction of a second. An

intensive generation of such microplasmas may have important implications for the

processes involved.


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The microwave heating of a dielectric material, which occurs through the conversion of

electromagnetic energy into heat within the irradiated material, offers a number of

advantages over conventional heating such as: (i) non-contact heating; (ii) energy

transfer instead of heat transfer; (iii) rapid heating; (iv) selective material heating; (v)

volumetric heating; (vi) quick start-up and stopping; (vii) heating from the interior of

the material body; and, (viii) higher level of safety and automation [3]. Due to these

advantages, microwaves are used in various technological and scientific fields in order

to heat different kinds of materials [2-4]. Most of the industrial applications of

microwave heating are based on heating substances that contain polar molecules, for

example: food processing, sterilization and pasteurization, different drying processes,

rubber vulcanization, polymerization or curing of resins and polymers by elimination of

polar solvents, etc. In addition, solid materials with a high dielectric loss factor,

i.e., microwave absorbers, can be subjected to different processes based on microwave

heating. Among these materials, carbons are, in general, very good microwave

absorbers, so they can be easily produced or transformed by microwave heating.

Moreover, carbon materials can be used as microwave receptors to indirectly heat

materials which are transparent to microwaves. Thus, carbon materials have been used

as microwave receptors in soil remediation processes, the pyrolysis of biomass and

organic wastes, catalytic heterogeneous reactions, etc. The high capacity of carbon

materials to absorb microwave energy and convert it into heat is illustrated in Table 1,

where the dielectric loss tangents of different carbons are listed. As can be seen, the loss

tangents of most of the carbons, except for coal, are higher than the loss tangent of

distilled water (tanδ of distilled water = 0.118 at 2.45 GHz and 298 K). The search and

compilation of these data is no a straightforward matter. Although this parameter is

helpful for the study of microwave heating, few research groups have determined the


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dielectric loss tangents of carbons and the data that can be found are scattered

throughout bibliography.

Table 1. Dielectric loss tangents for different carbon materials at a frequency of

2.45 GHz and room temperature, ca., 298 K

Carbon material tanδ = ε”/ ε’ Reference

Coal 0.02-0.08 [5, 6]

Carbon foam 0.05-0.20 [7]

Charcoal 0.11-0.29 [8, 9] Carbon black 0.35-0.83 [10, 11]

Activated carbon 0.57-0.80 [9, 12, 13]

Activated carbona 0.22-2.95 [14]

Carbon nanotube 0.25-1.14 [15, 16] CSi nanofibres 0.58-1.00 [17] a Activated carbon at a mean temperature of 398 K

The first commercial microwave oven was developed in 1952, although it was during

1970s and 1980s when the widespread domestic use of microwave ovens occurred, as a

result of Japanese technology transfer and global marketing [18]. Curiously, the

industrial applications of microwaves were initiated by the domestic ovens. However, in

recent years, the number of processes that combine the use of carbons and microwave

heating to obtain benefits with respect to other traditional methods based on

conventional heating has increased enormously. Thus, as can be seen from Figure 1, the

number of scientific publications related to these topics was very low until the late

1990s, but interest has risen drastically in the last decade and especially so in the last

five years.


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0

20

40

60

80

100

120

1970 1975 1980 1985 1990 1995 2000 2005Year

Nº o

f pap

ers

Figure 1. Evolution of the number of research papers published on microwave-assisted

processes involving carbon materials (Source: Scopus®).

The aim of this work is to review examples of different microwave-assisted processes

involving carbon materials. In these processes, microwave heating is used either to

produce or modify different carbon materials or is employed in combination with

carbons acting as microwave absorbers to enhance different processes in technological

applications. The amount of published work is relatively large, so an effort has been

made to be representative rather than comprehensive. Thus, the synthesis of a wide

range of carbon materials by microwave techniques is reviewed in section 2. Due to the

widespread use of activated carbons, their production, modification and regeneration is

treated in a separate section (section 3). The use of microwave heating in various

metallurgical and mineral processes is presented in section 4. Moreover, microwaves

can be used not only to treat and modify solid carbons but also for purposes of

revaluation, to obtain other products of high added value, such as gases. This is the case

of thermal valorisation of biomass and biosolids, which is dealt with in section 5.

Finally, the use of microwaves to enhance the reactions catalysed by carbons is growing

in importance and is discussed in section 6.


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2. Synthesis of carbon materials

Microwave plasma-enhanced chemical vapor deposition (CVD) has been widely used

for growing carbon nanotubes [19, 20] or diamond deposition [21]. Although very few

works have been described, the synthesis of nanocarbons by the direct microwave

irradiation of catalyst particles on a solid support is another possibility for producing

carbon nanofilaments by CVD [22, 23]. Thus, carbon nanofilaments were formed

through microwave-assisted CH4 decomposition over an activated carbon, acting as

catalyst and microwave receptor, using a combination of CH4/N2. The same method was

also used to grow carbon nanofilaments on carbon fibers doped with Fe and Ni [23],

giving rise to structures similar to those shown in Figure 2. Mixtures of CH4/CO2 were

also used to produce nanofilaments using a char as catalyst and microwave receptor [22].

Since no nanofilaments were observed after analogous tests carried out under

conventional heating, it must be concluded that it is the different mechanism involved in

heating the supports by microwaves that promotes the formation of nanofilaments.

Figure 2. Carbon nanofilaments grown on a carbon fiber. (Reprinted from Ref. [23] with permission from Elsevier).


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Carbon materials can act not only as a support for growing nanostructures but also as

the carbon-source for the synthesis of nanotubes and nanofibers via microwave heating.

It has been reported that is possible to synthesize these nanostructures by simply

microwave heating of graphite for 60 min [24] or of organometallic compounds like

ferrocene, Fe(C5H5)2, for just 30 min [25]. In addition, microwave irradiation seems to

be an effective option for purifying nanotubes. Thus, the refining of raw samples of

nanotubes, that is the elimination of the amorphous carbon formed during the synthesis

and metal impurities (the remains of the catalyst), has been carried out by microwave

heating, at 433 K for 30 min., in nitric acid. [26]. The microwave-induced

funcionalization of carbon nanotubes is another option. Reactions such as amidation and

1,3-dipolar cycloaddition have been performed in a matter of minutes, which is a

significant reduction in time compared to conventional funcionalization processes. In

addition, the number of steps involved in the reaction procedure has been reduced,

which opens the door to a fast and inexpensive process for producing functional

singlewall carbon nanotubes [27]. The multifuncionalization of carbon nanotubes using

a combination of two addition reactions, the 1,3-dipolar cycloaddition of azomethine

ylides and the addition of diazonium salts, has also been achieved in solvent free

conditions via a simple and fast microwave-induced method [28]. This method reduces

not only the reaction time but it also avoids the need for using toxic solvents. Another

example of the use of microwaves in the functionalization of carbon materials is the

oxidation of carbon nanohorns [29]. Furthermore, carbon nanotubes can be incorporated

into ceramic matrices using microwave irradiation. The use of a microwave-induced

reaction may be able to overcome some of the problems of the conventional methods of

synthesis, i.e., non-uniform dispersion and damage to the nanotubes during high

temperature processing in the reacting media. As an example, a carbon nanotube-silicon


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carbide composite has been synthesized by this method in only 10 minutes at

temperatures between 673 and 1173 K [30].

Another application of microwave heating is the production of graphitic carbons from

various precursors, such as isotropic or anisotropic pitch-based carbon powders [31].

Moreover, Mitsubishi Heavy Industries, Ltd. (MHI) has claimed (1) to have produced

superior-grade graphite from carbon powder by using a microwave process at 3473 K.

Expanded graphite, a modified graphite which retains its layered structure and is similar

to natural graphite but with larger interlaying spacing and abundant multipores, has

been produced by the microwave irradiation of both natural graphite, mixed with HNO3

and KMnO4 [32], and expandable graphite [33]. A microwave-based device specially

designed for the production of expanded graphite has been patented [34]. It has also

been claimed that powdered carborundum, i.e., silicon carbide, with diameters in the

nanometer range, can be produced by a microwave-assisted process that involves

reducing SiO2 with various forms of carbon in a nitrogen atmosphere [35]. In a similar

way, Changhong et al. used microwave heating to induce the direct reaction between

silicon and charcoal powder, at temperatures lower than 1273 K, in order to synthesize

silicon carbide [36].

Microwave irradiation has been used to synthesize ordered mesoporous carbon

materials (OMC) by employing a nano-casting technique and ordered mesoporous silica

as the template. Thus, microwave heating techniques have been applied to synthesize

nano-structured materials such as zeolites and ordered mesoporous silica materials

under hydrothermal conditions [37], the result being a remarkable enhancement in the

efficiency of sol–gel synthesis, as manifested by the shorter time or lower temperature


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required for the synthesis, and the unique and uniform properties of the products. The

ultrarapid production of OMC materials, which utilizes the intense heat generated by

microwaves during the carbonization step, offers a number of advantages, such as

energy saving, low temperature preparation (even at 673 K), controllable oxygen

content, i.e., a wide range of hydrophilicity without the need for post-activation, and an

enhancement of the conductivity of the carbon materials [38].

Another use of microwave heating in the synthesis of carbon materials is derived from

the capacity of microwaves to interact with polar solvents (Dipolar Polarization). In this

case, interaction can produce a more uniform dispersion of metal particles in the

synthesis of carbon-supported catalysts by means of the so-called microwave-assisted

polyol process [29, 39, 40]. Another advantage of the interaction of microwaves with

polar solvents is that they can be eliminated by evaporation, thereby saving both energy

and time. Thus, microwave heating can be used to facilitate the polymerization or

curing of polymers by eliminating solvents. The application of microwave irradiation to

polymer chemistry has also revealed potential advantages in its ability not only to drive

chemical processes but also to perform them in a short time [41]. A particular case of

industrial interest is the microwave-assisted vulcanization of rubber [42]. The use of

microwave heating in rubber vulcanization is now a well-established technique.

Interestingly microwave heating can also be used to devulcanize elastomer waste [43].

In the field of carbon materials, microwave heating has been applied in the drying step

of the synthesis process of carbon xerogels [44, 45]. Microwave drying has been used to

prepare resorcinol–formaldehyde gels by employing methanol [44] and water [45] as

solvents. By using microwave drying, the process for obtaining carbon gels has been

greatly simplified. As with other drying methods, the textural properties can be


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controlled, but time has been considerably reduced by changing from conventional

drying techniques to microwave drying (up to 90% reduction in the drying stage), no

pre-treatment being necessary. Taking into account that one of the disadvantages of

carbon gels, compared to other carbon materials, was the complexity, long time required

and the high costs of the traditional synthesis methods, this improvement (i.e., the use of

microwaves in the synthesis of carbon gels) makes these materials more economically

competitive in a large number of applications.

3. Production, modification and regeneration of activated carbons

Activated carbons are, in general, produced from different organic precursors, such as

biomass, coal, polymers, natural or synthetic fibres, etc, which are subjected to

carbonization and activation processes. The activation process can be carried out by

making steam, CO2 or air react with a char or a carbonized material at relatively high

temperatures, i.e., 873 to 1173 K. This kind of activation is commonly referred as

physical or thermal activation. On the other hand, chemical activation consists in the

simultaneous carbonization of the precursor with an activating agent, such as ZnO,

H3PO4, KOH, etc, at temperatures ranging from 673 to 1073 K. Both

microwave-assisted activation processes have been recently reviewed by Yuen and

Hameed [46].

Since carbonaceous materials are good microwave absorbers, the microwave-assisted

physical activation of chars from different organic precursors has been widely studied

over the last few decades. Norman and Cha reported a method for producing highly

microporous activated carbons by the CO2 activation of coal chars [47]. More recently,


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a research group from Kunming University of Science and Technology in China has

claimed to have obtained activated carbon by the steam activation of coconut shells in a

pilot-scale device of 60 kW with considerable saving of time and energy [48], whilst

another group from the University of Huddersfield in the United Kingdom has reported

activating a phenolic resin-derived carbon in air using constant rate microwave

thermogravimetry [49].

On the other hand, few attempts have been made to develop microwave-based processes

for chemical activation. The non-carbonized organic precursors of activated carbons

impregnated with activating agents are, in general, poor microwave absorbers. This

characteristic makes it difficult to heat them up with microwaves to the high

temperatures required, although this problem could be overcome by using microwave

receptors, e.g. carbonaceous materials [50]. Additionally, Yagmur et al. [51] reported a

microwave-induced impregnation method applied prior to the carbonisation stage

saving both time and energy.

A way of exploiting the excellent dielectric properties of carbon materials, i.e., their

high microwave absorption capacity, is the use of microwave heating to tailor their

surface chemistry either by introducing or by removing surface functionalities. As was

discussed in section 2, the microwave-assisted functionalization of carbon nanotubes

and nanohorns has already been successfully carried out. By contrast, in the case of

activated carbons, microwave heating has mainly been used for the rapid elimination of

surface functionalities that are often present on the carbon surface. Thus, microwave

heating has been used to remove oxygen functionalities and produce a highly-basic

activated carbon in just a few minutes, basic properties lasting upon air exposure [52].


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Similar results were obtained for carbon fibres subjected to thermal treatments in a

microwave oven [53, 54]. In the case of granular activated carbons, the main advantage

of employing microwave as opposed to conventional heating seemed to be the saving of

time and energy, as the chemical changes observed on the carbon surface did not differ

very much regardless of the method used, provided that similar temperatures were used

[55]. However, whereas in the case of the activated carbons the porous texture remained

more or less unaltered after microwave treatment, the carbon fibres subjected to

microwave treatments exhibited a reduction in micropore volume and micropore size.

This effect has been used to produce carbon molecular sieves (CMS) by subjecting

acrylic textile fibres to microwave action [56]. This novel method of producing and

improving the properties of molecular sieves of carbon fibres seems to be more

economical than the conventional one. In particular, CMS produced by this method

showed a very high selectivity for CO2/CH4 and O2/N2 gas separations [57].

Another topic of investigation has been the microwave-induced regeneration of

exhausted activated carbons. Thus, Price et al. [58] and Cha et al. [59] have reported

microwave-assisted regeneration processes of activated carbon saturated with volatile

organic compounds (VOC). As well as of carbons used in gas phase applications, the

regeneration of activated carbons used in liquid phase adsorption has been studied. Thus,

different activated carbons polluted with phenol [60] and pharmaceuticals [61] have

been subjected to thermal regeneration under N2 and CO2 atmospheres, using both

conventional and microwave heating. It was found that the rapid heating of the

exhausted activated carbons by microwave energy leads to a quick regeneration.

Furthermore, microwave technology allowed the activated carbons to be recycled and

reused several times. Thus, it was shown that microwave heating preserves the porous


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structure of the regenerated activated carbons more efficiently than treatment in a

conventional device. The use of this technique causes no damage to the carbon; rather, it

increased the surface area, at least during the first regeneration cycles [62]. Figure 3

illustrates the changes in the adsorptive capacity of a commercial activated carbon

exhausted with phenol, after subsequent cycles of regeneration at 1123 K in N2 and CO2

atmospheres, using both conventional and microwave heating. Interestingly,

regeneration in the microwave oven doesn’t just preserves (or even increases) the

adsorptive capacity of the activated carbon, the regeneration time is reduced by 90% as

well.

Figure 3. Comparison of the phenol adsorptive capacities of an activated carbon after

various cycles of regeneration: (N850EF) electric furnace at 1123 K in a N2 atmosphere;

(N850MW) microwave furnace at 1123 K in a N2 atmosphere; (C850EF) electric

furnace at 1123 K in a CO2 atmosphere; (C850MW) microwave furnace at 1123 K in a

CO2 atmosphere. Adapted from reference [60].

One more example of the application of microwave-assisted regeneration of activated

carbon is its use in certain gold recovery industries which is described in the next

section.


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4. Metallurgy and mineral processing

Microwave heating has been investigated for use in various metallurgical processes,

including pyrometallurgy, hydrometallurgy [63] and mineral processing [3].

Yet in the particular case of processes involving carbons, the microwave-assisted

reduction of metal oxides with different carbon materials has been extensively

investigated. Thus, hematite fines and magnetite concentrates have been mixed with

different carbons like charcoal or coke and then subjected to microwave heating up to

1273 K [64]. It was found that the process is influenced by the type of carbon used and

that, under similar conditions of temperature, microwave reduction produces better

results than those attained by conventional heating. Recently, a process for carrying out

the carbothermic reduction of iron oxides using microwave heating has been patented

[65]. Similar processes that combine microwave heating with the use of various kinds of

carbons as reducing agent and microwave absorbers are applied to other metallic

oxides [66].

Microwave heating is also employed in the gold mining industry to recover gold from

the activated carbon used in the so called carbon-in-pulp process (CIP). The carbon,

which is used to adsorb the gold cyanide molecule, is periodically removed from the

adsorption tanks to allow removal of the gold by elution. Then, after the carbon is

subjected to an acid wash to remove inorganic compounds, it is regenerated at

923-1123 K in a steam atmosphere to remove other foulants such as flotation reagents,

lubricating oils and humic acids which would foul the carbon and undermine its

performance. The regenerated carbon is then sized and returned to the CIP circuit.


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Regeneration is conventionally done in rotary kilns or vertical tube furnaces. However,

it has been demonstrated that the use of microwave heating can be also an alternative

economically feasible, with a saving of energy and time consuming [67].

Another field of application of microwave heating is the processing of coals. Coals are,

in general, very poor microwave absorbers (see Table 1), since they do not possess

graphene lattices of a size large enough to allow delocalized π-electrons to move in

order to couple with the electromagnetic field of the microwaves (i.e., heating by

interfacial polarization). Thus, only devolatized coals, chars or cokes, with a relatively

large amount of delocalized π-electrons that move on the incipient graphenic structures,

show good microwave absorbance properties (see Figure 4).

Figure 4. Representative coal molecular structure and char after devolatilization. Coal

doesn’t possess many delocalized π-electrons. After devolatilization aromatic rings fuse

given rise to small graphitic planes with π-electrons free to move.

Despite this, the microwave heating of coals is used in the industrial processing of coal.

For example, a process for the microwave-assisted grinding of coals has been described

[68]. In this process, coals of various ranks were exposed to microwave radiation to

CH4, CO2, CO, H2O, H2, SH2, HCN, … CH4, CO2, CO, H2O, H2, SH2, HCN, …


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quantify the effect on grindability. Reductions of up to 50% of relative grindability were

reported after 5 minutes of microwave exposure. These reductions are believed to be

due to fracture mechanisms; inherent moisture within the coal structure absorbs

microwaves changing phase, and producing considerable pressure and differential

expansion by gangue mineral components. Another example is the microwave-assisted

desulphurization of coals which is based on the fact that pyrite heats more rapidly than

coal. This heating has the effect of enhancing the magnetic susceptibility of pyrite

thereby improving the removal rates by magnetic separation [69]. In addition to these

two industrial applications of microwave heating of coals, the possibility of using

microwave heating of a high volatile bituminous coal for rapid coke making has been

studied at the bench scale [70]. In the cited work it is suggested that devolatilization

starts due to the moisture content and -OH bonds present in the coal, and then, when the

aromaticity of the devolatilized material increases, the absorption of microwaves allows

temperatures in excess of 1273 K, which produces graphitized coke in less than 2 h.

5. Thermal valorisation of biomass and biosolids

Microwave heating is a good alternative for carrying out the pyrolysis of biomass [71,

72], coal [73, 74], oil shale [75, 76], glycerol [77] and various organic wastes [78]. In

general, these materials are poor receptors of microwave energy, so they cannot be

directly heated up to the high temperatures usually required to achieve total pyrolysis.

However, microwave-induced pyrolysis is possible if the raw material is mixed with an

effective microwave receptor, such as certain metal oxides [73, 74] or carbon materials

[50, 71, 72, 75-78]. The latter are usually preferred for this particular application, not

only because they are very good microwave absorbers, but also because they are


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inexpensive, easily available in different textures, sizes, forms, etc; and do not add any

extra inorganic component to the solid residues obtained after pyrolysis. Moreover, the

carbonaceous residue itself which is obtained from the pyrolysis of the organic

materials can be used as an excellent microwave absorber [50, 71, 79, 80].

A case of particular interest is the pyrolysis of sewage sludge [50, 79-85], often called

biosolids. Handling of this waste, which can be considered as biomass, represents a

challenge in industrialized countries. So far, none of the methods used, from land

reclamation, such as landfill or organic fertilisers, to incineration, is exempt from

drawbacks, like collateral pollution or high costs of treatment. These high costs are in

part due to the need to remove the high water content of the sludge. Microwave heating

could be a highly efficient alternative for drying these biosolids [81]. Moreover, a

process that uses the steam produced by microwave drying to gasify the products

generated during subsequent pyrolysis has been described [80]. In this method, drying,

pyrolysis and gasification of the sewage sludge take place at the same time, giving rise

to a larger gas fraction with a high syngas (CO+H2) content [79, 84, 85] and to an oil

fraction with a low polycyclic aromatic hydrocarbons (PAHs) content [82, 83]. In

contrast, the oil from the conventionally heated sludge consists primarily of PAHs.

Unlike other conventional pyrolysis methods and due to the high temperatures that are

reached during the process, a partially vitrified solid residue can be obtained by

microwave-assisted pyrolysis [80]. This is particularly useful in the case of sludges with

high heavy metal content.

A unique characteristic of microwave-assisted thermal treatments of biomass and

biosolids is that they produce a considerable higher amount of H2 and CO (syngas) and


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much less CO2 than similar treatments carried out using conventional heating. This

feature is illustrated in Figure 5, where the compositions of the gas fractions obtained

from the pyrolysis of a biomass residue using both methods of heating are compared.

Given that microwave-assisted pyrolysis maximizes the gas fraction obtained (oils are

produced but in very small amounts) and the fraction of the carbonaceous residue can be

used as microwave receptor and consumed by auto-gasification with the CO2 obtained

in the process [72], this method can be used for the thermal valorisation of biomass and

biosolids, by producing mainly syngas-rich gases.

0

10

20

30

40

50

60

70

80

90

100

400 500 600 700 800 900 1000 1100

Temperature (ºC)

vol%

syngas EF syngas MWCO2 EF CO2 MW

Figure 5. Variation, with temperature, in the gas composition obtained from the

pyrolysis of a biomass residue using conventional (closed symbols) and

microwave-assisted (opened symbols) pyrolysis. Adapted from reference [72].

Microwave heating combined with the use of carbons as microwave receptors has also

been used in soil remediation to eliminate organic pollutants. Thus, the use of graphite

fibres, as microwave receptor, in microwave-assisted extraction (MAE) has been shown

to be a successful method for the extraction of contaminants in soils, rivers and marine

sediments [86]. Similar microwave-induced thermal treatments employing granular


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activated carbon as microwave absorber to achieve the appropriate temperature were

used to decontaminate the soil [87].

6. Microwave enhancement of carbon catalyzed reactions

Owing to their particularly strong interaction with microwave radiation and high

thermal conductivity, graphite and certain other carbons are efficient sensitizers. They

are capable of converting radiation energy to thermal energy, which is then transmitted

instantaneously to supported chemical compounds. Two types of reaction are favored

by carbons-microwave coupling: (i) reactions which require a high temperature, and (ii)

reactions involving chemical compounds which, like the organic compounds, have a

low dielectric loss and do not heat up sufficiently under microwave irradiation. Thus,

microwave heating is a valuable, non-conventional energy source for organic synthesis,

which can produce spectacular accelerations in many reactions [88]. Carbons and

carbon-supported catalysts are used as sensitizer or sensitizer-plus-catalyst in several

organic microwave-assisted synthesis reactions, such as Diels-Alder reactions, the

thermolysis of esters, the decomplexation of metal complexes, the pyrolysis of urea,

sterifications, etc. [89]. Carbon particles are used to selectively heat the catalyst and

substrate without bulk heating the solution. A case of particular interest is fullerene

chemistry under microwave irradiation [90], where fullerenes are subjected to different

organic reactions in order to functionalize them.

Carbons can be used as catalyst in a variety of heterogeneous gas-phase catalytic

systems [91]. Besides, they are good microwave absorbers. These two characteristics

have been combined to enhance various catalytic reactions, in which carbons play the


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double role of catalyst and microwave receptor. In fact, the interaction of microwave

irradiation with some heterogeneous catalytic systems has been proven to enhance the

reaction rates, the selectivities of the products and the conversion values [92-94].

NOx reduction [95, 96], SO2 reduction [97], catalytic CH4 decomposition for H2

production [98, 99] and CO2 reforming of CH4 (dry reforming) [100, 101] are just some

examples in which the use of carbon materials as catalyst and microwave receptor

resulted in a considerable improvement in the yield of the reactions. A variety of carbon

materials, such as activated carbons, metallurgical cokes, chars or anthracite, were used.

Several microwave-enhanced heterogeneous catalytic reactions, in which carbons acted

as catalyst and microwave receptor, are shown in Table 2.

Table 2. Examples of microwave-enhanced heterogeneous catalytic reactions, in which

carbons are used as catalysts and microwave receptors

Reaction Catalyst/microwave receptor Reference

C + 2NO → CO2 + N2 Subbituminous coal char, anthracite, metallurgical coke

[95-97]

C + NO → CO + 1/2N2 Subbituminous coal char, anthracite [95-97]

C + SO2 → CO2 + S Subbituminous coal char, anthracite [97]

2C + SO2 → 2CO + S Subbituminous coal char, anthracite [97]

C + CO2 ↔ 2CO Biomass char [72]

CH4 → C+ 2H2 Activated carbon [98, 99]

CH4 + CO2 ↔ 2CO + 2H2 Activated carbon [100, 102]

The improvement in the results is attributed to the way the catalyst is heated by

microwaves. Thus, in the case of microwave heating, the catalyst (i.e., the carbonaceous

material) is heated directly by the action of the microwaves, and so it is at a higher

temperature than the surrounding atmosphere. However, under conventional heating,


22

heat is transferred through the surface of the catalyst, mainly by conduction and

convection, resulting in a totally different temperature gradient. In addition to this, as

mentioned before, microplasmas are commonly produced during the microwave

irradiation of carbons. The temperature in these microplasmas (also called hot spots) is

believed to be much higher than the overall temperature of the system, which usually

favours heterogeneous reactions between the solid catalyst and the gases taking part in

the reaction. It should therefore be possible to apply this technique to other catalytic

reactions in which a carbon-based catalyst is used.

7. Conclusions

Carbon materials are, in general, very good microwave absorbers. This explains the

increasing interest over the last decade in using them in a wide variety of

microwave-assisted thermal processes. These processes include the synthesis of a wide

variety of carbon materials (i.e., nanostructures, graphite, active carbons, polymers, etc.),

the purification or even chemical and/or physical modification of carbons in a quick and

controlled way, the enhancement of different processes involving coal, chars or even

biomass/biosolids, and the clear improvement in the efficiency of some

carbon-catalyzed reactions. These processes are attracting considerable attention given

their possible use in commercial applications and some of them have already been

demonstrated at pilot or even industrial scale. The main advantage of using microwaves,

instead of traditional heating techniques, is because of the different mechanism involved

in heating carbons, leading to, (i) a considerable decrease in the time scale, which in

most cases implies a smaller consumption of energy, (ii) a reduction in the number of

steps involved in the global process, eliminating the need for other reagents, devices, etc.


23

and (iii) an increase in the efficiency of the global process. These advantages mean that

the final products obtained from microwave-assisted processes will probably be more

economically competitive than the ones obtained using traditional techniques.

Acknowledgements

B.F., Y.F and L.Z. are grateful to CSIC of Spain and the European Social Fund (ESF)

for financial support under thesis grant I3P-BDP-2006. Financial support from the

PCTI-Asturias (Project PEST08-03) is also acknowledged.

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